Motorized systems include both stationary mechanisms such as generators, pumps and the like, and moveable mechanisms, such as motor vehicles, including cars, truck, locomotives, and ships. Often times the motorized systems employ a power system including a selective catalytic reduction (SCR) device that converts NOx with the aid of a catalyst into diatomic nitrogen and water. In many cases, the SCR device relies upon ammonia as a reductant that is absorbed into the catalyst. Over time, the ammonia in the catalyst depletes and requires replenishment. Many systems employ a urea injector to add ammonia back to the catalyst. The urea injector is controlled to introduce a set amount of urea into the SCR device based on known conversion efficiencies. Existing urea injection controllers base urea injection on active NOx regeneration (interaction between NOx and the catalyst). However, in addition to active NOx regeneration, many SCR devices experience passive NOx regeneration, or regeneration that does not deplete ammonia storage. Accordingly, it is desirable to provide a urea injector controller that determines urea injection based on both active and passive NOx regeneration.